Eccentric feed rotation in zone refining

ABSTRACT

PROCESS OF ZONE MELTING A SEMICONDUCTOR ROD HAVING UNDER PROCESS, A RECRYSTALLIZED PORTION AND SUPPLY PORTION TO BE RECRYSTALLIZED, EACH SEPARATED FROM THE OTHER BY A MOLTEN ZONE, INCLUDES SUPPORTING THE ROD SUBSTANTIALLY VERTICALLY BY FIRST AND SECOND HOLDERS RESPECTIVELY LOCATED AT THE FREE ENDS OF THE SUPPLY ROD PORTION AND THE RECRYSTALLIZED ROD PORTION, FORMING THE MOLTEN ZONE IN THE ROD WITH AN ANNULAR HEATING DEVICE SPACED FROM AND SURROUNDING THE ROD, RELATIVELY MOVING THE ROD AND THE HEATING DEVICE IN A SUBSTANTIALLY VERTICAL DIRECTION SO AS TO PASS THE MOLTEN ZONE ALONG THE ROD, ROTATING AT LEAST ONE OF THE END HOLDERS ABOUT THE SUBSTANTIALLY VERTICAL AXIS OF THE ROD PORTION AT THE FREE END OF WHICH THE END HOLDER IS LOCATED, RELATIVELY MOVING THE END HOLDER TOWARDS ONE ANOTHER AND LATERALLY AWAY FROM ONE ANOTHER AT PREDETERMINED SPEEDS SO AS TO INCREASE THE THICKNESS OF THE RECRYSTALLIZED ROD PORTION IN A DIRECTION RADIALLY OUTWARDLY TO THE ANNULAR HEATING DEVICE AND ROTATING THE SUPPLY ROD PORTION ABOUT AN AXIS ECCENTRIC TO THE SUBSTANTIALLY VERTICAL AXIS OF THE SUPPLY ROD PORTION. APPARATUS FOR CARRYING OUT THE FOREGOING METHOD INCLUDES SUBSTANTIALLY VERTICAL SPACED END HOLDERS SUPPORTING BETWEEN THEM A VERTICALLY EXTENDING SEMICONDUCTOR ROD HAVING A RECRYSTALLIZED PORTION AND A SUPPLY PORTION TO BE RECRYSTALLIZED DIVIDED BY A MOLTEN ZONE, MEANS FOR RELATIVELY DISPLACING THE END HOLDERS TOWARD ONE ANOTHER, AN ANNULAR HEATING DEVICE SURROUNDING AND SPACED FROM THE ROD AND ADAPTED TO FORM THE MOLTEN ZONE IN THE ROD, MEANS FOR DISPLACING THE HEATING DEVICE ALONG THE ROD SO AS TO PASS THE MOLTEN ZONE ALONG THE ROD, MEANS FOR ROTATING AT LEAST ONE OF THE END HOLDERS ABOUT ITS SUBSTANTIALLY VERTICAL AXIS, MEANS FOR DISPLACING ONE OF THE END HOLDERS IN A DIRECTION TRANSVERSE TO THE AXIS THEREOF SO THAT THE RECRYSTALLIZED ROD PORTION IS FORMED WITH A SPECIFIC DIAMETER EXTENDING IN DIRECTION TOWARD THE ANNULAR HEATING DEVICE, AND MEANS FOR ROTATING THE SUPPLY ROD PORTION ABOUT AN AXIS ECCENTRIC TO THE AXIS OF THE SUPPLY ROD PORTION.

F8). 9, CARL-HEWZ VQGEL ET AL ECCENTRIC FEED ROTATION IN ZONE REFINING 2 Sheets-Sheet 1 Fig.3

CARL-HEINZ VOGEL ETAL 3,561,931

ECCENTRIC FEED ROTATION IN ZONE REFINING Feb. 9, 1971 2 Sheets-Sheet 2 Filed Aug. 4 1967 Fig. 5

United States Patent 0 U.S. Cl. 23301 6 Claims ABSTRACT OF THE DISCLOSURE Process of zone melting a semiconductor rod having under process, a recrystallized portion and a supply portion to be recrystallized, each separated from the other by a molten zone, includes supporting the rod substantially vertically by first and second holders respectively located at the free ends of the supply rod portion and the recrystallized rod portion, forming the molten zone in the rod wit-h an annular heating device spaced from and surrounding the rod, relatively moving the rod and the heating device in a substantially vertical direction so as to pass the molten zone along the rod, rotating at least one of the end holders about the substantially vertical axis of the rod portion at the free end of which the end holder is located, relatively moving the end holder towards one another and laterally away from one another at predetermined speeds so as to increase the thickness of the recrystallized rod portion in a direction radially outwardly to the annular heating device and rotating the supply rod portion about an axis eccentric to the substantially vertical axis of the supply rod portion.

Apparatus for carrying out the foregoing method includes substantially vertically spaced end holders supporting between them a vertically extending semiconductor rod having a recrystallized portion and a supply portion to be recrystallized divided by a molten zone, means for relatively displacing the end holders toward one another, an annular heating device surrounding and spaced from the rod and adapted to form the molten zone in the rod, means for displacing the heating device along the rod so as to pass the molten zone along the rod, means for rotating at least one of the end holders about its substantially vertical axis, means for displacing one of the end holders in a direction transverse to the axis thereof so that the recrystallized rod portion is formed with a specific diameter extending in direction toward the annular heating device, and means for rotating the supply rod portion about an axis eccentric to the axis of the supply rod portion.

Our invention relates to method and apparatus for crucible-free zone melting a crystalline rod, especially of semiconductor material.

In copending application Ser. No. 428,933 of W. D. Keller, filed Ian. 29, 1965, now abandoned, and assigned to the same assignee as that of the instant application, there is described a method of crucible-free zone melting wherein a semiconductor rod is vertically supported at one end by a first holder located in the vertical axis of the rod, a molten zone is formed in the rod by an annular heating device surrounding and spaced from the rod, and the rod is supported at its other end by a second rotatable end holder. The respective end holders are relatively movable in a direction toward one another and are laterally movable relative to one another, and the heating device is displaceable along the rod so as to pass the molten zone along the rod. One of the end holders is displaced ice laterally in a direction transverse to the vertical axis of the rod and of the annular heating device, the end holders being also respectively displaced in a vertical and similar direction so as to advance the first end holder toward the second end holder. The second end holder is rotated until the rod portion located between the second end holder and the molten zone is formed to a specific diameter which may be larger than the inner diameter of the annular heating device, then, after forming the rod portion to the specific cross section, the second end holder is displaced only in a vertical direction relative to the heating device. At the beginning of this known process, both holders of the rod are located in the same vertical axis. As the holder of the recrystallizing rod portion is laterally displaced relative to the holder of the supply rod portion to be crystallized and relative to the heating device, and the holders are simultaneously moved toward one another, the cross section of the rod portion recrystallizing from the melt continually increases. As soon as the nominal cross section of the recrystallizin rod portion is achieved, the end holder thereof is maintained in its eccentric position and is thereafter only adjusted with regard to its vertical position relative to the other holder. With this known process, semiconductor rods, preferably silicon rods, are able to be produced with greater cross sections than are able to be produced by the heretofore known coaxial zone melting process and have improved crystal quality and a relatively uniform radial resistance distribution over the rod cross section.

As a further improvement over the aforementioned method in copending application Ser. No. 428,933, it has been suggested in copending application Ser. No. 564,118 of W. Keller, filed July ll, 1966, now US. Patent 3,477,- 811 and assigned to the assignee of the instant application, to displace the recrystallizing rod portion again in the opposite direction and to repeat the lateral displacing motion during the zone melting operation several times in both directions. With this additional method step, the specific resistance distribution over the cross section of the rod can be made even more uniform.

In copending application Ser. No. 597,340 of W. Keller, filed Nov. 28, 1966 and assigned to the assignee of the instant application, an even earlier suggestion has been made that the semiconductor rod be end-supported by two end holders displaceable laterally and axially parallel to one another, one of the end holders being secured to a substantially vertical shaft. The shaft is mounted eccentrically in a cylinder rotatable about a substantially vertical axis in a bearing block. The cylinder is rotated by a worm drive, and the lower rod end holder is thereby eccentrically displaced. By means of a reversing motor, lateral reciprocatory motion is imparted to the lower rod end holder.

It has also been suggested in the aforementioned copending application Ser. No. 597,340 that the aforedescribed apparatus be improved by mounting the rotatable cylinder in turn in an eccentric rotatable in the boring block. Thus greater freedom is afforded in selecting the amplitude of the reciprocatory motion of the lower rod end holder.

It is accordingly an object of our invention to provide an eccentric zone melting process and apparatus for carrying out the process which improves over and considerably simplifies the aforedescribed eccentric zone melting process and apparatus.

With the foregoing and other objects in view, we provide in accordance with our invention process of zone rnelting a semiconductor rod having, under process, a recrystallized portion and a supply portion to be recrystallized, each separated from the other by a molten zone, which comprises supporting the rod substantially vertically by first and second holders respectively located at the free ends of the supply rod portion and the recrystallized rod portion, forming the molten zone in the rod with an annular heating device spaced from and surrounding the rod, relatively moving the rod and the heating device in a substantially vertical direction so as to pass the molten zone along the rod, rotating at least one of the end holders about the substantially vertical axis of the rod portion at the free end of which the end holder is located, relatively moving the end holders toward one another and laterally away from one another at predetermined speeds so as to increase the thickness of the recrystallized rod portion in a direction radially outwardly to the annular heating device and rotating the supply rod portion about an axis eccentric to the substantially vertical axis of the supply rod portion.

Apparatus for carrying out the foregoing method comprises substantially vertically spaced end holders supporting between them a vertically extending semiconductor rod having a recrystallized portion and a supply portion to be recrystallized divided by a molten zone, means for relatively displacing the end holders toward one another, an annular heating device surrounding and spaced from the rod and adapted to form the molten zone in the rod, means for displacing the heating device along the rod so as to pass the molten zone along the rod, means for rotating at least one of the end holders about its substantially vertical axis, means for displacing one of the end holders in a direction transverse to the axis thereof so that the recrystallized rod portion is formed with a specific diameter extending in direction toward the annular heating device, and means for rotating the supply rod portion about an axis eccentric to the axis of the supply rod portion.

Due to shifting the eccentric movement from the recrystallizing rod portion to the supply rod portion which is to be recrystallized, the mechanism serving for producing the eccentric motion is considerably simplified. It is of particular importance for the method of our invention that due to the stirring motion of the eccentrically rotated supply rod portion to be crystallized, the molten zone is well intermixed, producing highly uniform radial resistance distribution over the rod cross section. Further in accordance with a feature of our invention, both end holders are rotated in opposite rotary directions. A particularly good turbulence and intermixture of the melting zone is obtained, which is very desirable for obtaining good crystal quality and radial resistance distribution.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method and apparatus for crucible-free zone melting a crystalline rod, particularly of semiconductor material, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction of the apparatus and the method of the invention, however, together with additional objects and adavantages thereof will be best understood from the following description of a specific embodiment when .read in connection with the accompanying drawings, in which:

FIG. 1 is an elevational view partly broken away and partly in section showing two phases during the practice of the method of the invention;

FIG. 2 is an end view schematically showing the path of movement of the supply rod portion to be recrystallized;

FIG. 3 is an elevational view partly in section of a rod holder connecting the rod to a rotary shaft in accordance with the invention;

FIG. 4 is an upper end view of FIG. 3; and

FIG. 5 is a diagrammatic view of apparatus for carrying out the method of the invention.

Referring now to the drawings and first particularly to FIG. 1 thereof is shown a semiconductor rod 1 formed of a supply rod portion 2 which is to be melted and then recrystallized and a rod portion 3 that is resolidified or recrystallized from the melt. At the lower end of the rod portion 3 there is located a non-illustrated seed crystal which is fused thereto. An induction heating coil 4 is spaced from and surrounds the semiconductor rod 1 and is energizable with a high-frequency current from a nonillustrated source for producing a melting zone 5 separating the recrystallized rod portion 3 from the supply rod portion 2 to be recrystallized. The induction heating coil 4 can be moved upwardly or downwardly, as viewed in FIG. 1, along the semiconductor rod 1 or can remain stationary while the semiconductor rod 1 is moved upwardly or downwardly by moving respective end holders thereof (not illustrated in FIG. 1) so as to pass the molten zone 5 along the rod 1. The induction coil 4 serving as the heating device has a central axis M. A drive shaft 6 for the supply rod portion 2 that is to be recrystallized is eccentrically inserted in the holder 7 of the rod portion 2 and is secured in the eccentric, substantially vertical position shown in FIG. 3. The drive shaft 6 has a longitudinal axis E. Both rod portions 2 and 3 are rotated by drive mechanisms which are well known and are described hereinafter. At the same time, the recrystallizing rod portion 3 is laterally displaceable relative to the rod portion 2 and to the heating device 4. When the nominal cross section of the recrystallizing rod portion 3 is achieved, the relative lateral displacement thereof is discontinued and the rod portion 3 then only continues to rotate about its vertical axis A. The supply rod portion 2 that is to be recrystallized is axially moved opposite to the direction of movement of the heating device 4, as shown by the associated arrows at the right-hand side of FIG. 1, while the rod portion 3 need not be moved in the axial direction. It is of special advantage for the rod portions 2 and 3 to be rotated in opposite rotary directions about the axes A and E. The molten zone 4 is thereby well intermixed producing an exceptionally uniform radial resistance distribution over the rod cross section (Apz570%) The eccentricity e of the recrystallizing rod portion 3 with respect to the supply rod portion 2 should be so great that no liquid material drips from the molten zone 4. As an example, the eccentricity e can be substantially as much as 10 mm. for a recrystallizing rod portion 3 having a diameter of 35 mm. The eccentricity 2 of the rotary axis E of the supply rod portion 2 to the geometrical axis M thereof is adjustable advantageously by the holder 7 (FIG. 3) described more fully hereinafter. By varying the eccentricity e the stirring effect of the supply rod portion 2 is greatly varied. The additional area traversed by the supply rod portion 2, besides its own cross-sectional area, is shown by the hatching in FIG. 2.

The following data relate to the embodiment of the invention shown in FIG. 1:

The rotary speed of the recrystallizing rod portion 3 is between '8 and 70 rpm. for a rod diameter of 33 millimeters, and is preferably 25 r.p.m. The rotary speed of the rod portion 2 supplying the melt and which is to be recrystallized is between 8 and r.p.m. for a rod diameter of 27 mm., and is preferably 25 r.p.m. The rod portion 2 is advantageously moved .in a direction toward the rod portion 3 at a speed of about 1.5 mm. per minute. The heating device 4 is moved in a direction opposite to the movement of the rod portion 2 upwardly as shown in FIG. 1 at a speed of 1 to 2 mm. per minute and preferably 1.8 mm. per minute. The eccentricity 6 of the rod portion 2 is about 2 mm., whereas the eccentricity e of the rod portion 3 is set at about 8 mm.

An especially simple holder for the rod portion 2 utilized in carrying out the method of our invention, is

shown in FIGS. 3 and 4. The holder 7 is formed of two tubular portions 8 and 9 separated by a base or partition 10. The rod portion 2 is inserted into one tubular portion 8 and preferably clamped therein. Adjusting or set screws 13 are provided in two different planes 11 and 12 in the tubular portion 9 and serve for eccentrically holding the drive shaft 6 and for securing the rod portion 2 in vertical position. In the illustrated embodiment of 'FIG. 3, the adjusting screws 13 disposed in the plane 11 serve for establishing the eccentricity e of the drive shaft '6, whereas the adjusting screws 13 disposed in the plane '12 are provided for securing the rod portion 2 in vertical position. Preferably, three adjusting screws 13 circumferentially spaced 120 apart from one another are provided in each of the planes 11 and 12.

It is apparent that various steps of the method of our invention can be suitably altered without in any way departing from the basic concept of our invention and still remain within the scope thereof.

For example, both rod portions 2 and 3 can be axially displaced in the same direction at speeds that are predetermined with respect to one another in accordance with the desired rod dimensions. Furthermore, both rod holders can be rotated in the same rotary direction. The method of our invention is not limited to resolidifying or recrystallizing rod portions whose diameter is greater than the inner diameter of the heating device 4. It can, quite the contrary, be employed for producing recrystallizing rod portions having a diameter which is equal to or smaller than the diameter of the rod portion that is to be melted or recrystallized and/or the same as or smaller than the inner diameter of the heating device. Moreover, our method can also be instituted with advantage, when the recrystallizing rod portion is disposed coaxial to the center axis of the heating device. in addition, the employment of the method of our invention is advantageous if the diameter of both rod portions is larger than the inner diameter of the heating device.

The apparatus for carrying out the process of the invention is shown in FIG. 5.

A semiconductor rod, comprising the supply portion 2 and recrystallized portion 3 to which a seed crystal is fused, is vertically supported by the end holders 101, 102. A slider 103, displaceable on a rotary spindle 1104, has an extension 10 abutting the holder 101 so as to be able to displace the holder 101 and the rod portion 2 in either vertical direction depending on the direction of rotation of a reversible motor M A reversible motor M is supported on the extension 105 and drives a shaft 6 for rotating the holder 101 and the rod portion 2 about an axis E eccentric to the geometrical axis M of the rod portion. 2. The motor M is supported on a base 106 of the apparatus. The holder 102 of the recrystallized rod portion 3 is rotatable by a motor M which has a displaceable shaft at one end of which the holder 102 is secured. The motor M is fastened to a slide 107 which is horizontally displaceable by a rack and pinion mechanism driven by a motor M mounted on the base 106 whereby the holder 102 and the recrystallized rod portion 3 are displaceable in a direction transverse to the vertical axis of the rod. A motor M also mounted on the base 106, drives a rotary spindle 108 provided with a spindle head that is in engagement with the displaceable shaft of the motor M for vertically displacing the holder 102 and the recrystallized rod portion 3. The horizontal displacement distance of the recrystallized rod portion 3 is relatively small so that the relatively wide abutting surface of the head on the spindle 108 engages the displaceable shaft of motor M in all of the possible horizontally displaced positions of the slide 107 and the motor M secured thereto. An induction heating coil 4, preferably with a flat winding, as shown, surrounds and is spaced from the molten zone 5 and is vertically dis placeable by a slider 109 and a rotary spindle 110 driven by a motor M which is mounted on the base 106.

The apparatus of our invention is located in a vacuum or protective gas atmosphere in accordance with like apparatus described in Patents Nos. 2,972,525; 2,992,311 and 3,030,194.

We claim:

1. Process of zone melting a semiconductor rod having, under process, a recrystallized portion and a supply portion to be recrystallized, each separated from the other by a molten zone, which comprises supporting the rod substantially vertically by first and second holders respectively located at the free ends of the supply rod portion and the recrystallized rod portion, forming the molten zone in the rod with an annular heating device spaced from and surrounding the rod, relatively moving the rod and the heating device in a substantially vertical direction so as to pass the molten zone along the rod, rotating at least one of the end holders about the substantially vertical axis of the rod portion at the free end of which the end holder is located, relatively moving the end holders toward one another and laterally away from one another at predetermined speeds so as to increase the thickness of the recrystallized rod portion in a direction radially outwardly to the annular heating device and rotating the supply rod portion about an axis eccentric to the substantially vertical axis of the supply rod portion.

2. Process according to claim 1 including rotating said end holders in opposite rotary directions respectively.

3. Apparatus for carrying out the foregoing method including substantially vertically spaced end holders supporting between them a vertically extending semiconductor rod having a recrystallized portion and a supply portion to be recrystallized divided by a molten zone, means for relatively displacing the end holders toward one another, an annular heating device surrounding and spaced from the rod and adapted to form the molten zone in the rod, means for displacing said heating device along the rod so as to pass the molten zone along the rod, means for rotating at least one of said end holders about its substantially vertical axis, means for displacing one of said end holders in a direction transverse to the axis thereof so that the recrystallized rod portion is formed with a specific diameter extending in direction toward said annular heating device, and means for rotating the supply rod portion about an axis eccentric to the axis of the supply rod portion.

F holder of the supply rod portion by said adjusting ele ments in a position eccentric to the geometric axis of the supply rod portion.

6. Apparatus according to claim 3 wherein the eccentric axis is substantially parallel to the geometric axis of the supply rod portion.

References Cited UNITED STATES PATENTS 3,134,700 5/1964 Sporrer 23-301 3,191,924 6/1965 Haus 23273X 3,296,036 1/1967 Keller 23-301X 3,360,405 12/1967 Keller 23-301X 3,414,388 12/1968 Keller 23--30l NORMAN YUDKO'FF, Primary Examiner R. T. FOSTER, Assistant Examiner US. Cl. X.R. 

